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® Understanding Basic DMM Calibration For Electrical Calibration Sales 1 ® Chapter Objectives: Type of DMMs DMM Functional Block Diagram Analog to Digital Converter Types AC-DC Converters Resistance Converters Theory of DMM Calibration Calibration Strategies DMM Terminology 2 ® Is technical knowledge required? What if the cal procedure tests things in the wrong order? What if a parameter fails to meet the specification? What if the instrument requires troubleshooting? The technician who understands the technology is an asset to the lab! 3 ® Types of DMMs Laboratory DMMs Handheld DMMs 4 Bench DMMs ® Types of DMMs Laboratory DMMs 6 1/2 to 8 1/2 digits resolution Usually 5 functions DC Volts AC Volts DC Current AC Current Resistance May be configured for IEEE remote operation 5 ® Types of DMMs Bench/System Meters Less accurate than laboratory DMMs Generally 4 1/2 to 6 1/2 digit resolution Generally same 5 basic functions May be configured for IEEE remote operation Depending on model, may be software calibrated 6 ® Types of DMMs Handheld DMMs Most versatile of the DMMs, but least accurate Generally 3 1/2 or 4 1/2 digit resolution Basic 5 functions, but may also include: Frequency Continuity Diode Test Peak hold Temperature Capacitance 7 Waveform ® DMM Terminology Function - Functional options of a DMM. e.g. DC Voltage, AC Voltage, Resistance, Current, Frequency, etc. Range - A DMM's measurement capability is split into ranges such that the input signal can be scaled to a level appropriate for its ADC or RMS converter. Resolution & Scale Length -- A way of describing how many figures a DMM can display. Resolution is the number of digits e.g. 1.000 000 V is a 6½ digit display where the ½ digit is the "1" and there are 6 places after the decimal point. Sensitivity is the smallest measurable amount (Lowest Significant Digit) on any range. 8 ® DMM Terminology Linearity - Linearity is a description of how the gain of an instrument's ADC might vary with the amplitude of the measured signal. Zero or Offset - All DC measurements are affected by residual Offsets that may be in Volts, Amps or Ohms. Frequency Response -- Frequency Response is interpreted as the -3dB point i.e. the high frequency point at which the reading has reduced to 70.7% of its nominal low frequency value. A more meaningful measure of response is Flatness. Flatness is a measure of the deviation from an assumed flat reference level. 9 ® DMM Terminology Input Resistance - Is important because it describes the potential loading effect of the DMM on the test circuit. For DC voltage up to 20 V, the input resistance may be >1012 W. At higher voltages up to 1 kV, an attenuator is switched in to divide the applied voltage down to 10 V or 1 V. The attenuation resistor is typically 10 MW. For AC voltage, the input resistance will usually be lower – typically 1 MW with 150 pF in parallel. 10 ® DMM Terminology Input Bias Current – Amplifiers are usually designed for high gain, high bandwidth, high input impedance, low output impedance and low input voltage offsets. Bi-polar semiconductor devices require an input current to "bias" them into a desirable (usually linear) portion of their operating curve. There will be some residual input current, which will flow in the external measurement circuit. A small current of 10 pA in 1 MW will develop 10 µV. A modern sensitive DMM would be expected to have an input bias current of <50 pA. 11 ® DMM Terminology Crest Factor - Is the peak-to rms ratio capability of a DMM's AC converter. A pure sine-wave has a crest factor of 1.414:1, a square wave has a crest factor of 1:1. 12 ® DMM Terminology Common Mode Rejection - A measure of how the DMM responds to the presence of a particular kind of interference (unwanted) signal. Common mode errors can be reduced by careful guarding techniques to reduce the effects of unwanted leakages to ground. Normal Mode Rejection - A measure of how the DMM responds to an interference signal that is not common to both inputs. 13 ® DMM Terminology Guard - A guard is a circuit that may be used to intercept and divert, or control leakage currents. It may be passive, where it is simply a conductive screen or case around sensitive components and circuits, or active where amplifiers actively sense and control the screen potential. When line powered instruments are connected together, circulating leakage currents may flow around the "loop" formed by their respective power supplies and input lead connections. 14 ® DMM Terminology Guard - A guard is a circuit that may be used to intercept and divert, or control leakage currents. Such currents usually manifest themselves as noisy or inflated readings in sensitive measurements and may be eliminated by connecting the guard of the DMM to the "earth" terminal (not directly to earth) of the "source" instrument. 15 ® DMM Terminology 4 Wire Resistance – A measurement technique used to minimize the effects of the resistance of the connecting leads. In a 4-wire resistance measurement, separate conductor pairs are used to connect the current and voltage circuits. True RMS - In high accuracy AC voltage measurement, the most useful description of the amplitude is the Root-Mean-Square value. The term True RMS was coined to distinguish between modern RMS sensing instruments and the older or lower cost DMMs that used mean or average sensing, but were adjusted to indicate the RMS value. 16 ® DMM Terminology Calibration Uncertainty - The total uncertainty of the calibration standard traceable to national and international standards. The uncertainty will include the contribution of the DMM during the calibration process. Relative Accuracy – The accuracy of the DMM relative to but not including calibration standards. Absolute Accuracy - The combination of Calibration Uncertainty and Relative Accuracy. The combination may be a simple arithmetic summation of the two terms, or a more complex Root-Sum-of-Squares (RSS) combination at a specified confidence level. 17 ® DMM Block Diagram 18 A Simplified Block Diagram of a DMM ® DMM Block Diagram The ADC is the heart of the DMM 19 A Simplified Block Diagram of a DMM ® DMM Block Diagram Measurement Section Multifunction Analog Input Display Control Section Electrically Isolated Communication and Control Channels Computer Interface IEEE-488 A. TYPICAL BLOCK DIAGRAM 20 RS-232 Typical computer Interface ® DMM Block Diagram Measurement Section Multifunction Analog Input Display Control Section Electrically Isolated Communication and Control Channels Computer Interface IEEE-488 A. TYPICAL BLOCK DIAGRAM 21 No calibration required RS-232 Typical computer Interface ® DMM Block Diagram Measurement Section Display Multifunction Analog Input Control Section Electrically Isolated Communication and Control Channels Computer Interface IEEE-488 A. TYPICAL BLOCK DIAGRAM 22 RS-232 Checked during power up, or with self Typical test computer Interface ® Multifunction Analog Input DMM Block Diagram DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 23 OHMS Ohms Reference ® Multifunction Analog Input DMM Block Diagram DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 24 OHMS The ADC is the heart of the DMM. All measurement functions go through the ADC Ohms Reference ® Multifunction Analog Input DMM Block Diagram DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Ohms Reference Ohms OHMS B. TYPICAL MEASUREMENT SECTION 25 OHMS An ADC is usually a single range circuit, and it can handle only a narrow voltage range of zero to ±10 V ® Multifunction Analog Input DMM Block Diagram DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Ohms Reference Ohms OHMS B. TYPICAL MEASUREMENT SECTION 26 OHMS Circuits in the form of amplifiers and attenuators scale the input voltage to levels that can be measured by the ADC ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 27 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Reference Stability (% of Reading) Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 28 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference ADC Linearity (% of Scale) Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 29 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Attenuator Stability (% of Reading) Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 30 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Voltage Offsets (Absolute) Current Shunts DCI DCI & ACI Ohms OHMS B. TYPICAL MEASUREMENT SECTION 31 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Input Bias Current (Absolute) Ohms OHMS B. TYPICAL MEASUREMENT SECTION 32 OHMS Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Noise Ohms OHMS B. TYPICAL MEASUREMENT SECTION 33 OHMS (Absolute) Ohms Reference ® Multifunction Analog Input Contributions to the Spec DCV DC Volt Ranges & Filtering DCV ALL OTHERS A to D Converter (ADC) To Control Section for Display. ACV AC Range Amp and AC-DC Converter ALL OTHERS ACI ACV & ACI DC Reference Current Shunts DCI DCI & ACI Resolution (Absolute) Ohms OHMS B. TYPICAL MEASUREMENT SECTION 34 OHMS Ohms Reference ® Contributions to the Spec 35 ±%R ±%FS ±µV ® Contributions to the Spec The basic DC linearity needs only to be verified on the basic (10V) range 36 ±%R ±%FS ±µV ® Contributions to the Spec The 100 mV and 1 V ranges will have a slightly worse performance. Noise and voltage offsets will be the dominant factor 37 ±%R ±%FS ±µV ® Contributions to the Spec The 100 mV and 1 V ranges will have a slightly worse performance. InNoise the higher voltageoffsets ranges, and voltage willthe effects power dissipation be theofdominant factor The basic DC linearity needs in the attenuators will be the dominant only to be verified onfactor. the basic (10V) range 38 ±%R ±%FS ±µV ® Contributions to the Spec In the higher voltage ranges, the effects of power dissipation in the attenuators will be the dominant factor. 39 ±%R ±%FS ±µV ® Analog-to-Digital Converter (ADC) S3 S2 VRef • S1 Vmeas OP Amp Integrator De-integration Process – S2 closed, S1 and S3 Open – Vref applied to Cint in opposite polarity via Rin – Cint discharges to 0V – Time for discharge monitored by counter • Zero Process – S3 closed, S1 and S2 Open 40 Cint Integration process – S1 closed, S2 and S3 Open – Cint charges to conditioned voltage level “Vmeas” via Rin • Rin Used in low resolution DMMs Larger Vmeas Vref discharges Cint at same rate Smaller Vmeas Variable Charge Slope Constant Discharge Slope Constant Time - T1 Variable Time - T2 Basic Dual-Slope Converter Zero Volts Comparator ® DC Voltage Converter Amplifiers Or Attenuators Scale the input to the proper level for the ADC Amplifies the low input ranges Attenuates the high input ranges May include filtering circuits 41 ® Alternating Voltage (AC to DC) Converters Range amplifier attenuates or amplifies input to proper range for conversion Applied to AC to DC Converter DC output applied to ADC for digitizing ACVmeas AC to DC Converter Range Amp 42 ADC ® Main Types of Alternating Voltage Converters Average Responding Converter True RMS Voltage Converters Thermal Sensing Technique Log/Antilog Analog Computation Digital Sampling ACVmeas AC to DC Converter Range Amp 43 ADC ® Average Responding Converter Operational Rectifier ADC and DMM Display ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 44 ® Average Responding Converter The Average Responding Converter must have a SINE wave input. Operational Rectifier ADC and DMM Display ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 45 ® Average Responding Converter Operational Rectifier ADC and DMM Display ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 46 The Operational Rectifier determines the average after rectification ® Average Responding Converter Operational Rectifier ADC and DMM Display ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 47 The Average (0.637 of peak) is scaled up by 1.11 to RMS equivalent ® Average responding AC-DC Converters Converts the AC input signal to an equivalent DC voltage Usually the average voltage is converted to the corresponding RMS value through multiplication by a constant The conversion is made with the assumption that the applied signal is pure sine-wave Otherwise the conversion factor is not correct and the meter indicates an incorrect value For a pure sine wave the conversion is as follows: Vrms 48 2 π Vavg 1.11 Vavg 4 ® True RMS Converters The function of a True-RMS converter is to convert the AC input signals to a DC voltage value proportional to the RMS value of the input signal True-RMS values are obtained mathematically by averaging the squared value of the input, then finding the square root of the average square 49 ® True RMS Voltage Converters Three general types of “True RMS” Converters Thermal Sensing Log/Anti-Log Analog Computation Digital Sampling 50 ® Thermal Sensing Technique RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 51 ® Thermal Sensing Technique The Range Amp scales the basic operating range up or down RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 52 ® Thermal Sensing Technique RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common The resistor heats the transistor, increasing conduction 53 ® Thermal Sensing Technique RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 54 Collector current increase causes voltage to decrease, unbalancing differential amplifier ® Thermal Sensing Technique RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 55 The differential amplifier changes current through resistor, heats transistor until balance is achieved ® Thermal Sensing Technique The differential amplifier output potential level is sensed to ADC RMS Sensor DMM Display ADC ACVmeas Range Amp Capacitance Voltage is Proportional to Average Current from Range Amp. DMM Common 56 ® Log/Antilog Analog Computation 57 ® Log/Antilog Analog Computation A True RMS converter consists of a precision rectifier, Logarithmic and Exponential amplifiers. 58 ® Log/Antilog Analog Computation A True RMS converter consists of a precision rectifier, Logarithmic and Exponential amplifiers. The logarithmic and Anti-log amplifiers effectively square and square-root the input signal 59 ® Log/Antilog Analog Computation A True RMS converter consists of a precision rectifier, Logarithmic and Exponential amplifiers. The output of the multiplier Vx has a DC ripple content that is averaged (to obtain a mean value) by applying it to an active low-pass filter 60 ® Log/Antilog Analog Computation A True RMS converter consists of a precision rectifier, Logarithmic and Exponential amplifiers. Vout is also used to provide the feedback signal Vf, which provides the square-root element of the computation and controls the gain of the circuit to 61 give a wide dynamic range ® Log/Antilog Analog Computation A True RMS converter consists of a precision rectifier, Logarithmic and Exponential amplifiers. 62 The low-pass filter determines the low frequency response of the instrument. Some log-feedback DMM's are capable of operating down to 0.01 Hz. ® Digital Sampling In this technique, portions of the input waveform are sampled during short intervals of time relative to the period of the waveform. Each sample is quickly digitized by a fast ADC. Its value is squared by the DMM’s microprocessor and the result is stored in its memory. When a sufficient number of samples have been taken, the CPU takes the square root of the average value of the squares. Waveform Sampling ACVmeas Sampling Gate Range Amp 63 Microprocessor Fast ADC ADC and DMM Display DMM COMMON ® Digital Sampling Mathematically the rms value of samples in the sampling technique is expressed as: N Vrms 2 V i i 1 N Where: 64 N = Total number of samples Vi = i th voltage sample i sample number such that 1 i N = ® Resistance Converters 65 ® Resistance Converters Two different resistance conversion schemes are commonly employed in today’s DMMs The constant current method Ratio method 66 ® Constant current method of Resistance Conversion 67 ® Constant current method of Resistance Conversion In the constant current method, constant current sources, are built into the DMM. DMM 19.9992 Range Amp ADC Precision Constant Current Source in DMM Rmeas DMM COMMON 68 The DMM measures the voltage across this resistor. This constant current source is applied through Rmeas, the unknown resistor. ® Ratio Resistance Measurement 69 ® Ratio Resistance Measurement Vref from Rstd S2 S3 Rstd Floating Current Source in DMM Rin Buffer Amp Cint Rmeas Internally, an approximately known current is applied through Rmeas, the unknown resistor. S1 Vmeas from Rmeas It is connected in series to a known reference resistor Rstd. A simple voltage 70 divider is formed. 0V Vmeas from Rmeas Operation Integrator Vref from Rstd Zero Volts Comparator ® Ratio Method of Resistance Conversion The unknown resistance value can be determined by solving: Rmeas 71 Vmeas Rstd Vstd ® Current Converters 72 ® DMM Current Converters + Iin R1 RShunt R2 Shunt Current Converter E out Either the dc or ac voltage section senses the voltage across the shunt Iin R shunt (R1 R 2 ) R2 Two methods are commonly used for DMM Current Converters 73 ® DMM Current Converters + Ifb Iin R1 - E out - Iin R1 + RShunt R2 Feedback Current Converter An unknown current flows to an operational amplifier circuit which generates a proportional output voltage Shunt Current Converter E out Iin R shunt (R1 R 2 ) R2 E out Iin R f Two methods are commonly used for DMM Current Converters 74 ® DMM Current Converters + Ifb Iin R1 - E out - Iin R1 + RShunt R2 Feedback Current Converter Shunt Current Converter E out Iin R shunt (R1 R 2 ) R2 E out Iin R f Two methods are commonly used for DMM Current Converters 75 ® General Calibration Requirements Regardless of the type of the DMM, calibration adjustments are performed to reduce instability in the offset, gain and linearity of the transfer functions of the signal processing units Each of the functional blocks in a DMM’s measurement section is subject to these sources of variation in performance 76 ® Theory of Calibration Adjustment Assume that all of the DMM’s signal processing blocks have a transfer function of: y = mx + b Where: y = the output m = the gain x = the input b = the offset 77 ® Theory of Calibration Adjustment y = mx + b The DMM is designed in such way that ‘m’ has an exact nominal value, such as 10, and ‘b’ should be zero (i.e.; no zero offset). When ‘m’ is not equal to its nominal value, all values of ‘y’ will deviate from nominal by the same percentage. 78 ® Theory of Calibration Adjustment y = mx + b When ‘b’ is not equal to zero, all values of ‘y’ will be offset from their correct value by a constant amount. 79 ® Contributions to the Spec Display + Full Reading 0 - Full Reading 0 Input 80 ±%R ±%FS ±µV ® Contributions to the Spec Display + Full Reading 0 - Full Reading 0 Input 81 Even though ‘m’ and ‘b’ deviate from their nominal values, the slope of ‘y’, as a function of ‘x’, should be a straight line. ±%R ±%FS ±µV ® Theory of Calibration Adjustment A DMM operates under this y = (mx + b) relationship 82 ® Theory of Calibration Adjustment It takes the unknown input x (whether in voltage, current or resistance) and converts it into y (in Volts, Amperes or Ohms) 83 ® Theory of Calibration Adjustment Voltmeter specifications are based primarily on offset, gain and linearity The % of reading specification is the gain uncertainty “Zero” uncertainty is due to voltage offsets in the ADC amplifiers and comparators, specified in the specifications as the range error, or the floor Linearity uncertainty is due to the secondary error sources, such as mismatches in resistive ladders of the ADC, or dielectric absorption errors by storage capacitors 84 ® Theory of Calibration Adjustment In addition to these sources of uncertainty, some DMMs have spurious voltage spikes, DC bias currents, and pump-out currents present at their input terminals These can introduce errors in the measurements 85 ® Internal References Every DMM has a voltage reference The DC voltage is used as the reference for the ADC and is the limiting factor for the DMM’s best accuracy for all the voltage and current measurements, and therefore also the resistance measurements There are also one or more resistance references that determine the accuracy of the resistance converter 86 ® Direct Voltage Reference + 15 V Power +7 V Main Reference - 1 Gain 0 Volts REF AMP -7 V Main Reference - 15 V Power 87 ® Resistance Reference Reference Current + - 54.6k - 10V From Precision Voltage Reference 88 Precision resistor along with precision dc voltage gives a known resistance current. ® DC Voltage Calibration Strategy Functional Tests Prior to starting the calibration process: Run Self – Test if available Check each function and range using a copper short or known voltage or resistance or current 89 ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence 90 10 V Zero, +10 V Gain, -10 V Gain 10 V Linearity: ±1 V, ±5 V, ±15 V, ±19.9 V 1 V Zero, +1 V Gain, -1 V Gain* 100 mV Zero, +100 mV Gain, -100 mV Gain* 100 V Zero, +100 V Gain, -100 V Gain* 1 kV Zero, +1 kV Gain, -1 kV Gain* ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence The 10V range is the first one measured. 91 10 V Zero, +10 V Gain, -10 V Gain 10 V Linearity: ±1 V, ±5 V, ±15 V, ±19.9 V 1 V Zero, +1 V Gain, -1 V Gain* 100 mV Zero, +100 mV Gain, -100 mV Gain* 100 V Zero, +100 V Gain, -100 V Gain* 1 kV Zero, +1 kV Gain, -1 kV Gain* ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence 92 10 V Zero, A short is used to establish the zero +10 V Gain, -10 V Gain 10 V Linearity: ±1 V, ±5 V, ±15 V, ±19.9 V 1 V Zero, +1 V Gain, -1 V Gain* 100 mV Zero, +100 mV Gain, -100 mV Gain* 100 V Zero, +100 V Gain, -100 V Gain* 1 kV Zero, +1 kV Gain, -1 kV Gain* ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence 93 10 V Zero, +10 V Gain, -10 V Gain 10 V Linearity: 1 V Zero, The of 1 V and ±1 V, ±5 V, ±15 V,lower ±19.9ranges V 100 mV would be measured next +1 V Gain, -1 V Gain* 100 mV Zero, +100 mV Gain, -100 mV Gain* 100 V Zero, +100 V Gain, -100 V Gain* 1 kV Zero, +1 kV Gain, -1 kV Gain* ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence 94 10 V Zero, +10 V Gain, -10 V Gain 10 V Linearity: ±1 V, ±5 V, ±15 V, ±19.9 V 1 V Zero, +1 V Gain, -1 V Gain* 100 mV Zero, Then the ranges of 100 V and +100 mV Gain, -100 mV Gain*be measured 1 kV would 100 V Zero, +100 V Gain, -100 V Gain* 1 kV Zero, +1 kV Gain, -1 kV Gain* ® DC Voltage Calibration Strategy A Typical DC Voltage Calibration Sequence 10 V Zero, +10 V Gain, -10 V Gain 10 V Linearity: ±1 V, ±5 V, ±15 V, ±19.9 V 1 V Zero, +1 V Gain, -1 V Gain* 100 mV +100 mV Gain, -100 mV Gain* To test forZero, linearity errors in the range amplifier, the voltage is applied at both + and – range 100 V Zero, +100 V Gain, -100 V Gain* limits 95 1 kV Zero, +1 kV Gain, -1 kV Gain* ® AC Voltage Calibration Strategy The AC functions have the added dimension of frequency. This complicates calibration by introducing additional test points for each amplitude range. The AC converter module must provide gain and attenuation for a wide range of signals from typically a few millivolts to 1 kV The gain of a DMM's AC function will vary with frequency. This is known as its Frequency Response and requires that measurements are made at key points throughout each amplitude range. 96 ® AC Voltage Calibration Strategy Frequency Response 97 ® AC Voltage Calibration Strategy Frequency Response From 40 Hz to 5 kHz, the DMM's gain is determined by resistor networks, its AC to DC converter and DC reference 98 ® AC Voltage Calibration Strategy Frequency Response At higher frequencies, reactive effects, primarily capacitive, will determine the flatness of the DMM's frequency response 99 ® AC Voltage Calibration Strategy A Typical AC Voltage Calibration Sequence 100 1 V AC Zero, ±1 V DC Turnover* Crest Factor: (at 3:1 or 5:1 as required)* 1 V LF Gain, 1 V HF Gain, Check Frequency Response* 1 V LF Linearity, 1 V HF Linearity* 100 mV Zero, response* 100 mV LF, 100 mV HF, Check Frequency 10 V Zero, 10 V LF, 10 V HF, Check Frequency Response* 100 V Zero, Response* 100 V LF, 100 V HF, Check Frequency 1 kV Zero, 1 kV LF, 1 kV HF, Check Frequency Response* *These adjustments will usually be iterative. Always follow the manufacturer's recommendations for the adjustment sequence. ® AC Voltage Calibration Strategy A Typical AC Voltage Calibration Sequence 1 V AC Zero, ±1 V DC Turnover* Crest Factor: (at 3:1 or 5:1 as required)* 1 V LF Gain, 1 V HF Gain, Check Frequency Response* 1 V LF Linearity, 1 V HF Linearity* 100 mV Zero, response* 10 V Zero, 100 V Zero, Response* 1 kV Zero, 101 The first adjustment is DC Turnover, where the gain of the 100 mV LF, 100 mV HF, Check Frequency precision rectifier is adjusted to be identical for both positive and negative excursions of the input 10 V LF, 10 V HF, Check Frequency Response* signal 100 V LF, 100 V HF, Check Frequency 1 kV LF, 1 kV HF, Check Frequency Response* *These adjustments will usually be iterative. Always follow the manufacturer's recommendations for the adjustment sequence. ® AC Voltage Calibration Strategy A Typical AC Voltage Calibration Sequence 1 V AC Zero, ±1 V DC Turnover* Crest Factor: (at 3:1 or 5:1 as required)* 1 V LF Gain, 1 V HF Gain, Check Frequency Response* 1 V LF Linearity, 1 V HF Linearity* 100 mV Zero, response* 10 V Zero, 102 The second is Crest Factor, which canFrequency be affected by non100 mV LF, 100 mV HF, Check symmetry of the rectifier and also by the bias arrangements of the 10 V LF, 10 V HF, Check Frequency Response* analog multiplier 100 V Zero, Response* 100 V LF, 100 V HF, Check Frequency 1 kV Zero, 1 kV LF, 1 kV HF, Check Frequency Response* *These adjustments will usually be iterative. Always follow the manufacturer's recommendations for the adjustment sequence. ® AC Voltage Calibration Strategy A Typical AC Voltage Calibration Sequence 1 V AC Zero, ±1 V DC Turnover* Crest Factor: (at 3:1 or 5:1 as required)* 1 V LF Gain, 1 V HF Gain, Check Frequency Response* 1 V LF Linearity, 1 V HF Linearity* 100 mV Zero, response* AC zeroFrequency measurements are 100 mV LF, 100 mV HF, Check 10 V Zero, 100 V Zero, Response* 1 kV Zero, 103 often made with a short-circuit applied to the DMM's input, but 10 V LF, 10 V HF, Check Frequency Response* some instruments will require a smallFrequency AC bias voltage because 100 V LF, 100 V HF, Check the RMS converter cannot operate with a zero input 1 kV LF, 1 kV HF, Check Frequency Response* *These adjustments will usually be iterative. Always follow the manufacturer's recommendations for the adjustment sequence. ® Resistance Calibration Strategy The accuracy of the resistance function is dependent upon both the resistance current source and the DC voltage ranges of the DMM The resistance function is usually measured after the DC voltage function Changes in the performance of the either the current defining resistors or voltage gain-defining resistors will affect the resistance calibration For sensitive long-scale DMMs, it is usually better to start with the high resistance ranges and then work down. This is will allow a longer thermal stabilization time for the connecting leads and the DMM's internal circuits 104 ® Resistance Calibration Strategy 105 ® Resistance Calibration Strategy The resistance function of a DMM consists primarily of a constant current source providing a range of currents typically from 10 nA to 10 mA. 106 ® Resistance Calibration Strategy Selecting a resistance range selects an appropriate constant current to pass through the unknown or standard resistance. 107 ® Resistance Calibration Strategy The voltage developed across the resistance is then measured by the DMM's DC voltage function 108 ® Resistance Calibration Strategy Resistance Linearity Tests For DMMs with a resistance function accuracy of not better than 100ppm (4½-5½ digit resolution), a decade resistor box could be used To measure linearity on the 10 kW range of a 7½ digit DMM with a maximum indication of 19.000 000 kW would require several different resistance standards 109 ® Resistance Calibration Strategy If the DC Voltage range is linear, then the resistance ranges are also linear The resistance linearity can be checked by inserting a resistance in series with the I+ lead and verify the current source can deliver the same current through a range of resistance 110 values . ® Current Calibration Strategy Therefore changes in the DC or AC 100 mV range performa nce will also affect the current ranges From Fig 10-14 112 • The current ranges are linked together. • The shunts are connected in series with suitable tapping points for the voltage sensing • To measure 0.1 mA, the current is passed through all of the shunts in series and their values will be chosen such that the sum of their resistive values for any particular current range will develop 100 mV for the specified current The voltage developed across the shunts will be passed to the DC or AC 100 mV ranges depending upon whether the DCI or ACI function has been selected. All of the shunts included will have an affect on that range's performance ® Current Calibration Strategy A Typical Current Calibration Sequence DC Current Calibration Sequence 100µA Zero, +100µA, -100µA 1mA Zero, +1mA, -1mA 10mA Zero, +10mA, -10mA 100mA Zero, +100mA*, -100mA* 1A Zero*, +1A*, -1A* Minimize any heating effects by measuring the lowest current ranges first *These measurements may require a longer settling time due to self-heating and thermo-electric effects. 113 ® Current Calibration Strategy A Typical Current Calibration Sequence AC Current Calibration Sequence 100µA LF Gain, 100µA HF Gain** 1mA LF Gain, 1mA HF Gain** Minimize any heating effects by measuring the lowest current ranges first 10mA LF Gain, 1mA HF Gain** 100mA LF Gain, 1mA HF Gain** 1A LF Gain, 1A LF Gain** ** Some DMMs do not have an HF adjustment 114 ® Zero Considerations • All DC functions will need their zero offsets evaluated and compensated before measurements of the gain values are made • Zero is only a baseline for the measurements and it is the gain values that transfer traceability • It is important that all DC measurements are referred to a known offset state, but that state does not have to be exactly zero • An offset will affect all readings by a fixed amount 115 ® Zero Considerations • It is very important that a "system" zero is performed when a DMM is being calibrated. - A system zero means zeroing the DMM to the zero offset of the calibration standard. • For voltage measurements, the DMM should be zeroed to the source "zero" output. • For resistance measurements, the DMM should be zeroed to the resistor such that the effects of Voltage offsets in the Hi and Lo circuit are removed. • Current measurements require the DMM to be zeroed in the open-circuit state, although some calibration sources have a "zero" current output to which the DMM should be zeroed. 116 ® DMM Measurement Uncertainty Measurement Uncertainty Contributions • Uncertainty of the Standard • Resolution of the DMM • Short-Term Stability of the DMM with Time and Temperature • Combined Noise of Standard and DMM • The Calibration Procedure 117 ® DMM Uncertainty Uncertainty of the Standard • It is imperative that the calibration standard is of sufficient accuracy to be able to calibrate the DMM with confidence • Sometimes Test Accuracy Ratios (TAR) are quoted as a requirement i.e. a TAR of 4:1 requires the accuracy of the standard to be 4 times better than the specification of the DMM 118 ® DMM Uncertainty The Resolution or Scale • The resolution of the DMM could become a limiting factor in the measurement regardless of how accurate the calibration standard might be Short-Term Stability • The short-term stability of the DMM (and the calibration standard) with time and temperature will also affect the uncertainty of measurement • Usually the dominant factor is the temperature coefficient of the DMM and stability of the calibration environment • Secular drift is not usually significant unless the calibration takes several hours or the instrument is defective. 119 ® DMM Uncertainty The Combined Noise of the Standard and DMM • The combined effect of the noise of the standard and the DMM can be an important factor • If individual readings can be easily observed, and if the noise is predominantly random, the sample standard deviation of the readings can be calculated and used as the uncertainty contribution for combined noise. 120 ® DMM Uncertainty The Calibration Procedure • A poorly chosen test sequence, insufficient settling time or poor interconnection techniques will all introduce additional errors that may pass unnoticed by the operator • The manufacturer's recommended procedure should be used as the basis for DMM calibration. Note that this also applies to the use of the calibration standards. 121 ® Understanding Basic DMM Calibration For Electrical Calibration Sales Congratulations – you have completed this section of ECAL Competency Training 122